1,720,972 research outputs found
Book review: International Relations: The Key Concepts, 2nd edition by Martin Griffiths, Terry O'Callaghan and Steven C. Roach
No full textBook review: International Relations: The Key Concepts, 2nd edition by Martin Griffiths, Terry O'Callaghan and Steven C. Roac
Lateral resistance of "rigid" pipelines and cables on rocky seabeds
Full text linkAccurate assessment of lateral resistance is critical to ensure the on-bottom stability and integrity of subsea pipelines and cables in the oil/gas and marine renewable energy industries. However, on rocky seabeds recommend practices provide limited recommendations on pipe/seabed interaction, suggesting only a single value for the friction coefficient of 0.6. This paper reports on a programme of physical experiments and theoretical modelling investigating the lateral resistance of pipes on rocky seabeds. It is shown that the peak and mean effective friction can significantly exceed the interface (or Coulomb) friction coefficient when the pipe diameter (D) is similar to the median rock diameter (dn50). Only when the pipe diameter becomes large compared to the rock size does the mean effective friction approach the interface friction. The effective friction coefficient was found to vary with variability in rock size and shape, as well as the length of pipe relative to median rock diameter. Each of these findings is reproduced well using the theoretical model and demonstrating that the effective lateral friction coefficient maybe higher than 0.6 for mean friction, and significantly higher for peak friction. This implies that inaccuracy may exist in current design, which may be rectified using the theoretical model
Stability of subsea pipelines during large storms
No full textpipeline stability, scour, offshore hydrodynamics Author for correspondence
Progress in investigating pipe-soil-fluid interaction: The STABLE pipe JIP
No full textThe subsea pipeline industry has recognised for over two decades that sediment transport, scour and liquefaction mechanisms have potentially significant impact on the validity and accuracy of existing pipe-soil interaction models under extreme metocean conditions. This is highlighted in the recently-published DNV-RP-F109 code in Sections 4.4-4.5. Appendix B of the code also discusses the need for specific models to be used in calcareous soils. The STABLEpipe JIP is presently in Phase 2, with the objectives of undertaking research activities which could achieve a step change in the science and engineering of subsea pipeline stability design. A summary of the proposed work and present status of the JIP will be presented.</p
Lateral resistance of pipes on rocky seabeds: comparison between measurements and models based on synthetic seabeds
No full textThe behavior of pipelines, cables and umbilicals on rocky seabeds has to date received little research attention. This is despite the marine renewable energy and oil and gas industries relying on these 'pipes' to cross a variety of rocky seabed types in the presence of extreme metocean conditions. Present design solutions are challenging and costly, yet there remains a track record of in-service failures. This paper forms part of a wider research effort being undertaken by the University of Western Australia (UWA) into pipe behavior on rocky seabeds. This work includes the effects intermittent gaps have on hydrodynamic forces, the effect of seabed roughness on enhanced boundary layer thickness and the validity of existing hydrodynamic force models for small diameter cables. In this paper, the lateral resistance of pipes on rocky seabeds is investigated using both physical and numerical testing of model pipes over artificially-created rocky seabeds. Four model pipes of varying diameter have been displaced laterally over 1 m square model rocky seabeds, with a range of pipe to rock diameters. The lateral resistance of the physical pipe tests were recorded using load cells and a digital data-logger. Analysis of the physical test results has enabled comparison to (and refinement of) numerical models as well as improved understanding of the importance of different parameters. Our results show peak frictions above 6 arising under conditions where interface friction is only about 0.3, which contrasts dramatically with the friction value of 0.6 nominated in F109. This work contributes towards generation of new design methods suitable for application to field conditions.</p
Modelling changes to submarine pipeline embedment and stability due to pipeline scour
No full textIn this paper a beam bending model is combined with existing predictive formulas for pipeline scour to study changes to pipeline stability during scour and lowering. The model is introduced and demonstrated for a range of simplified conditions, including scour-induced lowering of a pipeline resulting from multiple uniformly spaced scour initiation points. The model is then used with a synthetic seabed generated with a variety of length scales. In this simulation the pipeline is 'laid' onto the seabed, leading to the formation of 'natural' initiation points for scour. The distribution and spacing of the initiation points (which are a function of the pipeline bending stiffness, tension and seabed roughness) lead to different rates of pipeline lowering and stability. The resulting model may be used within a probabilistic framework to estimate changes to pipeline stability resulting from sediment mobility and scour.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Observed changes to the stability of a subsea pipeline caused by seabed mobility
No full textHigh resolution bathymetry combined with structural modelling is used to estimate changes in the on-bottom stability of an offshore pipeline due to scour and sedimentation over an 11 year period. Detailed observations of post-lay embedment changes have been combined with the pipeline structural characteristics and an elastic-plastic model of soil resistance to estimate the vertical and horizontal stability of the pipeline using a finite difference solution to the beam bending equation. Application of the design approach indicates that post-lay increases to the critical (break-out) velocity of 1 – 2 m/s occur along the full 19 km of surveyed pipeline due to scour and sedimentation, which act to reduce load and increase soil resistance. The rate at which this increase in stability occurs with time is found to vary along the pipeline, and is dependent on the mechanism of pipeline lowering (i.e. whether the pipe lowered due to sagging into widely spaced scour holes, or by sinking into the shoulders between many closely spaced scour holes). By incorporating sediment transport into the pipeline design, the present results suggest potential for significant improvements in pipeline on-bottom stability and associated reductions in minimum required specific gravity and/or secondary stabilisation
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